1. Field of the Invention
The present invention relates to a phenanthrene compound and a light-emitting element including the phenanthrene compound. The present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Further, since such a light-emitting element can be formed to have a film shape, plane light emission can be easily obtained. Therefore, a large-area element capable of the plane light emission can be formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to a lighting device and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, by application of voltage to a light-emitting element, electrons and holes are injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. Then, these carriers (i.e., electrons and holes) are recombined, whereby the light-emitting organic compound is excited. The light-emitting organic compound returns to the ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound that is capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) generally exhibits only luminescence from the singlet excited state (fluorescence) and does not luminesce from the triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis of S*:T*=1:3.
On the other hand, when a compound in which a triplet excited state is converted into luminescence (hereinafter, such a compound is referred to as a “phosphorescent compound”) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency can be 3 times to 4 times as much as that of the fluorescence compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element (e.g., see Non-Patent Document 1).
When a light-emitting layer of a light-emitting element is formed using the above phosphorescent compound, the light-emitting layer is formed so that the phosphorescent compound is dispersed throughout a matrix formed of another material in many cases, for suppression of the concentration quenching of the phosphorescent compound and the quenching due to triplet-triplet annihilation. In this case, the material used for forming the matrix is referred to as a host material, and the material dispersed throughout the matrix is referred to as a guest material.
When a phosphorescent compound is used for a guest material, a host material is required to have higher triplet excitation energy (a difference in energy between the ground state and the triplet excited state) than the phosphorescent compound. It is known that CBP, which is used as the host material in Non-Patent Document 1, has higher triplet excitation energy than a phosphorescent compound which emits light of green to red and is widely used as a host material for the phosphorescent compound.
However, although CBP has high triplet excitation energy, it has insufficient ability to receive holes or electrons, which results in a problem of an increase in driving voltage. Therefore, a substance which has high triplet excitation energy and also can easily accept and transport both holes and electrons (i.e., a bipolar substance) is needed as a host material for a phosphorescent compound.
Furthermore, since singlet excitation energy (an energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore, a substance which has high triplet excitation energy and a bipolar property as described above is also effective as a host material in a light-emitting element using a fluorescent compound as a light-emitting substance.